The invention relates to a controlling device for the water-tank of a flush toilet. The conventional two-level controllable water-tank, the controlling components of which are all fitted on the hydraulic backing gate, has the disadvantage of being extremely laborious to operate because of larger force is required to be applied to the handle to open the hydraulic backing gate. Thus there often occurs a malfunction of a broken chain and the floating ball for adjusting the high and low water levels and its fasteners often influence the normal operation of the feeding float and the feeding velocity. In addition, the modification of controlling devices of these water-tanks is difficult and can only be completed by special tools and professional persons, making the manufacturing and modification cost too high to be easily and widely accepted by the market.